Membrane Permeability

Question 1

What molecules can pass freely across the plasma membrane?

Answer 1

Small uncharged polar molecules (water, urea, glycerol)

Hydrophobic molecules and gases (o2, CO2, n2, benzene)

Question 2

If Gibbs free energy △G is positive the diffusion is _________

Answer 2

Active - change in free energy

Question 3

If Gibbs free energy △G is negative the diffusion is _________

Answer 3

Passive

Question 4

What is active transport?

Answer 4

Allows movement of ions/molecules against their concentration or electrical gradient (unfavourable)
Energy required indirectly or directly from ATP hydrolysis

Question 5

Which cells have high energy expenditure on active transport?

Answer 5

Nerves
Muscle
Erythrocytes
Enterocytes

Question 6

What are the general intracellular and extracellular Na concentrations?

Answer 6

Outside 145 mM

Inside 12 mM

Question 7

What are the general intracellular and extracellular K concentrations?

Answer 7

Outside 4 mM

Inside 155 mM

Question 8

What are the general intracellular and extracellular Cl concentrations?

Answer 8

Outside 123 mM

Inside 4 mM

Question 9

What are the general intracellular and extracellular Ca concentrations?

Answer 9

Outside 1-2 mM

Inside 100 nM

Question 10

What is uniport?

Answer 10

Only one type of molecule transported at a time

Question 11

What is cotransport?

Answer 11

More than one ion/molecules transported at a time

Question 12

What is symport?

Answer 12

Cotransport in the same direction

E.g. Na/Glucose cotransported in small intestine and kidney

Question 13

What is antiport?

Answer 13

Cotransport in opposite directions

E.g. NCX.

Question 14

What is the function of the Na/K ATPase?

Answer 14

Active transporter present in every cell
Transports 3Na out of cell and 2K into cell per ATP molecule hydrolysed

Acts to maintain cellular concentrations of Na and K (form the gradients)

Question 15

What channels are responsible for resting membrane potential (~-70mV) ?

Answer 15

K diffusion through K+ channels

Question 16

What is the main function of the K/Na ATPase?

Answer 16

Form sodium and potassium gradients which are necessary for electrical excitability

Also derived secondary active transport (cotransport)

Question 17

How does the Na/k ATPase regulate ph?

Answer 17

Sodium gradient generated by Na/k ATPase is used to exchange H+ ions out of the cell (1:1 exchange)

Question 18

What is high intracellular toxic to cells?

Answer 18

Causes precipitation of phosphate (ossification) and Pi can’t be recycled

Question 19

What is the role of mitochondrial Ca uniporters?

Answer 19

At high cytoplasmic calcium, operate as a buffer to take calcium into mitochondria (facilitated diffusion), induces caspase pathway which induces apoptosis of the cell (cell death at high [Ca2+]

Question 20

In control of pH which two exchangers are important in lowering acid (H+ leaves)?

Answer 20

• Na/H exchanger (NHX) - one Na in, one H out

- Na dependent Cl-/HCO3- exchanger - one Na in , 1 H out, 1 HCO3 in, 1 Cl out

Question 21

What does electroneutral mean?

Answer 21

The number of positive/negative charges leaving is equal to the the charge entering

E.g. Na/H exchanger

Question 22

What is the Na/H exchanger activated and inhibited by ?

Answer 22

Activated by growth factors

Inhibited by amiloride (K+ sparing diuretic)

Question 23

In control of pH which exchanger is important in lowering base inside cell (HCO3- leaves)?

Answer 23

• Cl/HCO3- anion exchanger - one HCO3- out, 1 Cl in

Important in peripheral circulation

Question 24

What molecules are important in cell volume regulation?

Answer 24

Water follows osmotically active ions such as Na, K, Cl (6 molecules of water move across membrane per one ion) and organic osmolytes (such as amino acids) into/ out of cell causes shrinking or swelling

Question 25

What mechanisms resist cell swelling (remove water from cells)?

Answer 25

Conductive systems - facilitated diffusion of K and Cl out of cell (no change in electronegativity), water follows

Cotransport systems - H/K exchanger and cl/HCO3- exchanger lead to H + HCO3 = H2CO3 -> CO2 + H2O leave cell
- K/Cl cotransport out of cell, water follows

Question 26

What mechanisms resist cell shrinking (add water to cells)?

Answer 26

Conductive systems - facilitated influx of Na or Ca, water follows

Cotransport systems - influx of H2O and CO2 formation of H2CO3 which forms H (leaves cell, Na enters) and HCO3- (leaves cell, cl enters)
- Na/Cl cotransport into cell, water follows

Question 27

In an average human (70kg), how much of the body is water?

Answer 27

60% water

60% of 70 = 42 L water

Question 28

How much of the body’s water is extracellular?

Answer 28

1/3 of the body’s water is extracellular (2/3 intracellular)

So in a 70kg person, 42 L of water - 14 L extracellular
- 28 L intracellular

Question 29

Of the extracellular water, how much is interstitial and how much is in plasma?

Answer 29

In a 70kg individual, 14L extracellular water

11L interstitial water
3L plasma

Question 30

What is the circulating blood volume of a 70kg individual?

Answer 30

5L circulating blood volume

• 3L plasma
• 2L red cells

Question 31

What are the consequences of an increase in the permeability of blood capillaries to plasma proteins?

Answer 31

Plasma proteins leave capillaries which lowers oncotic pressure in the capillaries, enters interstitial space, increases hydrostatic pressure, water follows causes odema

Question 32

What are the glucose transporters on the intestinal epithelial cells?

Answer 32

On the lumen face of the epithelial cells - Na/glucose symporter (uses Na gradient, to transport glucose from intestine into cells)

Between epithelial cell and capillary, GLUT2 which transports glucose out of cells into capillaries

Question 33

Where in the body are Na/glucose symporters found?

Answer 33

Intestinal cells

Kidney epithelial cells

Question 34

What glucose transporter is present in adipose, brain, liver and skeletal cells?

Answer 34

Have an insulin receptor that upon insulin binding, causes insertion of glucose channels into the plasma membrane (via exocytosis of vesicles containing channels) allows glucose influx into cells

Question 35

How is glucose that enters cells prevented from leaving?

Answer 35

Converted into either glycogen (storage) or G6P (respiration - glycolysis)

Question 36

What other organic molecules use Na gradient for transport unfavourably up gradient?

Answer 36

Vitamin C

Amino acids